1. Field of the Invention
This invention relates to a method for producing corrugated cardboard sheets used for packaging or cushioning purposes, wherein all or part of the individual layers comprising the corrugated sheet are made of paper.
2. Description of the Prior Art
Corrugated cardboard sheets have been widely used for packaging purposes because of economy and ease of handling and transportation and because they possess a number of the requisite properties of a protective covering material.
Heretofore, a variety of types of corrugated cardboard sheets have been proposed and produced which are made from various materials, such as paper, plastics and other materials. In particular, a corrugated cardboard made of plastic is called a "plastic cardboard".
As is well known in the art, a typical construction of such corrugated cardboard sheets includes a corrugated core sheet bonded on both or either side to a flat-faced liner sheet. In the most simplified form, only one flat faced liner sheet is combined with the corrugated core sheet although a plurality of liner sheets may be combined with one or more core sheets to enhance the strength and cushioning effect.
Corrugated cardboard sheets using paper, however, have poor resistance to water and also are lacking in rigidity, and these factors mitigate against wider use of such corrugated cardboard sheets. This problem may arise due to the use of an aqueous starch paste as the adhesive for bonding the core sheet and the liner sheets since the use of an aqueous starch paste requires that the paper material possesses a sufficient water absorbing capability to facilitate bonding by the aqueous starch paste.
Recently, with increases in the operational speed of the corrugating machine, use of a paper material of a type having a higher degree of water absorption capability is desirable and this contributes to a lack of water resistance and rigidity in the finished product.
In order to improve the water resistance of the corrugated cardboard sheets using paper as the main component, attempts have been made to use, as the adhesive, plastic emulsion adhesives, aqueous polyvinyl alcohol adhesives, resorcin-formaldehyde denatured starch paste, hot-melt adhesives and the like.
However, the methods and the products of the prior art are not still satisfactory from the standpoint of water resistance, ease of handling and performance.
Another approach which has been used to improve the water resistance of corrugated cardboard sheets involves the application of wax coating to the faces of the corrugated core sheet and also the faces of the liner sheets. However, most of the commercially available water proofing corrugated cardboards are produced by dipping a sheet assembly of a core sheet and liner sheets which have been adhesively bonded with a water proofing starch paste into liquid wax to form coatings on the surfaces of the sheet assembly, and in using this process the wax coating does not penetrate to a sufficient extent into the portions of the sheet assembly corresponding to the surfaces of the core sheet meeting the liner sheets bonded by the starch paste.
Furthermore, the adhesive layer of starch paste itself does not possess sufficient resistance to water. As a result, water which passes between the surfaces of the core sheet meeting the liner sheets can easily penetrate into the starch paste layer and then into the paper sheets comprising the corrugated cardboard. This results in a drastic reduction of strength or, in the extreme case, a complete destruction of the cardboard structure.
Thus, prior art corrugated cardboard sheets using paper have limited resistance to water and also have some other inherent problems with respect to their physical properties.
A corrugated cardboard sheet using plastic is known and disclosed, for example, in U.S. Pat. No. 3,723,333. The corrugated cardboard sheet disclosed in this patent comprises a corrugated core sheet and one or more flat faced liner sheets both of which are made of plastic, the plastic core sheet being adhesively bonded to one of the respective plastic liner sheets with a plastic resin polymer having a melting point at least 10.degree. C lower than the melting point of the plastic polymer comprising the core sheet and that of the liner sheets, thus providing a corrugated cardboard structure free of "warp" and "crookedness".
While a plastic cardboard sheet of this type provides better resistance to water, this plastic cardboard sheet does not provide to a sufficient degree the requisite properties of good cushioning or as a packaging material such as rigidity, compression strength, burst strength, printability and processing property as does a corrugated cardboard sheet using paper and is also too expensive to be a substitute for paper cardboard.